Method of reducing particulate emissions

ABSTRACT

The present invention relates to lubricating oils, and in particular to the use of lubricating oils with low sulphur content in combination with a low sulphur fuel to reduce particulate emissions of a diesel engine equipped with a particulate trap. Thus, there is provided the use of an engine lubricating oil having a low sulphur content in combination with a fuel having a low sulphur content, to reduce the emissions of nucleation mode particles from a diesel engine fitted with a particulate trap. There is also provided a method of reducing the number of nucleation mode particles in the emissions from a diesel engine fitted with a particulate trap, which method comprises using an engine lubricating oil having a low sulphur content in combination with a fuel having a low sulphur content.

The present invention relates to lubricating oils, and in particular to the use of lubricating oils with low sulphur content in combination with a low sulphur fuel to reduce particulate emissions of a diesel engine equipped with a particulate trap.

Diesel engines are commonly used on private and commercial vehicles, particularly on commercial vehicles such as buses and lorries. It is known that emissions from diesel engines may comprise carbon oxides, nitrogen oxides, sulphur oxides, hydrocarbons and particulates. It is desirable to reduce these emissions either as a whole or individually. Whilst some of the emissions have their origin in the fuel which is combusted in the engine, the lubricating oil which is used to lubricate the engine can also impact on the tail-pipe emissions, for example by direct emission of combustion products of the oil or by affecting the trap performance.

In particular, the particulate emissions from an engine are believed to be related, at least in part, to the sulphur content of the fuel. Thus, in addition to the benefit lower sulphur gives to after-treatment devices, there has been a trend in recent years to reduce sulphur content of internal combustion fuels.

Despite the trend towards low sulphur fuels, with the advent of increasingly stringent particulate emissions controls in many areas of the world, for example, in the EU and USA, such as the particulate emissions limits for vehicles within city limits in states such as California, and states in the north-east of the USA, there may be a requirement for diesel vehicles to be fitted with particulate traps.

Particulate traps have been shown to be effective at trapping particles formed in the combustion process. During the combustion process, and especially in the presence of an oxidation catalyst in a catalysed particulate trap, a percentage of the sulphur in the fuel forms sulphates. Where a particulate trap is present the majority of this should remain in the particulate trap. However, under certain operating conditions, where the temperature of the trap becomes elevated, this material is released and, along with volatile emissions that now come straight through the trap, can condense after the trap to produce large numbers of nucleation mode particles.

These, extremely small, nucleation mode particles typically have a diameter of 30 nm or less, such as in the range of from 1 nm to 30 nm inclusive, for example in the range of from greater than 3 nm to 30 nm inclusive. Although larger carbonaceous particles (accumulation mode particles) make up the majority of the mass of particulate emissions, whilst the nucleation mode particles make up a relatively low mass of particulate emissions, it has been found that these nucleation mode particles can make a significant contribution to the total number of particulates emitted.

It is thus desirable to reduce the number of these nucleation particles emitted.

We have now surprisingly found that the concentration of nucleation mode particle emissions from a diesel engine fitted with a particulate trap may be significantly decreased by use of an engine lubricating oil having a low sulphur content (low sulphur lube oil) in combination with a fuel having a low sulphur content (low sulphur fuel).

Thus, according to the present invention there is provided the use of an engine lubricating oil having a low sulphur content in combination with a fuel having a low sulphur content, to reduce the emissions of nucleation mode particles from a diesel engine fitted with a particulate trap.

It has been found that use of a low sulphur lube oil with a low sulphur fuel according to the present invention causes significantly reduced nucleation mode particulate emissions compared to use of a conventional lube oil with a low sulphur fuel. Surprisingly the reduction in nucleation mode particulate emissions is significantly larger than might be expected based on the reduction in sulphur level of the lube oil alone.

Thus, according to another embodiment of the present invention there is provided a method of reducing the number of nucleation mode particles in the emissions from a diesel engine fitted with a particulate trap, which method comprises using an engine lubricating oil having a low sulphur content in combination with a fuel having a low sulphur content.

The present invention is particularly useful wherein the particulate trap is a catalysed particulate trap, which comprises both an oxidation catalyst and a filter. An example of such a trap is a continuously regenerating trap (CRT™). In the combustion of a fuel the majority of any sulphur present is converted to sulphur dioxide, with a relatively small amount, typically 1-2%, being converted to sulphates. These sulphates may act as precursors for particulate formation. In the presence of a particulate filter, but the absence of an oxidation catalyst, the gas formed from combustion of the fuel (and lube oil) contacts the filter, which will remove at least some of the particles formed from the gas. However the trapped particles may quickly block the filter, and to burn the particles off (as CO₂) requires very high temperatures, not normally reached in the trap. In a catalysed particulate trap, as well as the filter there is also provided an oxidation catalyst. The gas first contacts the oxidation catalyst, wherein, for example, components such as sulphur dioxide in the gas are oxidised to sulphates. The oxidised gas then contacts the filter, which can trap the particulates. In a continuously regenerating trap, at least some of the particulates trapped are burnt off from the filter by reaction with oxidation products from the catalyst, such as nitrogen dioxide (which is formed by oxidation of NO_(x) species in the combustion gas). These reactions occur at lower temperatures than those that would otherwise be required to bum the particulates off, and at temperatures that can be reached in the traps fitted to diesel engines, and hence the trap is continuously regenerated. However, sulphates are not burned off, but are re-volatilised at high temperatures, thus providing the potential to re-form as particles post-trap.

The diesel engine may be any suitable diesel engine but is preferably a heavy duty diesel engine.

The low sulphur fuel preferably has a sulphur content below 100 ppm (by weight), such as below 50 ppm. More preferably the sulphur content of the fuel is below 20 ppm, and most preferably is 10 ppm or lower.

The low sulphur lube oil preferably has a sulphur content of less than 0.4% (by weight), such as less than 0.3%. More preferably the lube oil has a sulphur content of less than 0.2%, and most preferably less than 0.15%.

A known additive used in lubricating oils for lubricating diesel engines engine is zinc dialkyl dithiophosphate (ZDDP). This is used as an anti-wear, anti-oxidant and corrosion inhibitor additive. However, this additive contains sulphur. Therefore according to another aspect of the present invention the lubricating oil has a ZDDP content at most 0.8% by weight, preferably at most 0.4% by weight, and more preferably is substantially free of ZDDP.

The lubricating oil may comprise one or more anti-wear additives which might be used, at least in part, to replace ZDDP, such as anti-wear additives selected from the group consisting of (a) molybdenum containing compounds, such as molybdenum dithiocarbamate (MoDTC), molybdenum dithiophosphate and molybdenum amines (b) organic based friction modifiers, such as oleamides, acids, amines, alcohols, phosphate esters and glycerol monooleates, and (c) salicylate-type detergents, such as calcium salicylate and magnesium salicylate.

The lubricating oil may comprise one or more anti-oxidant additives which might be used, at least in part, to replace ZDDP. Preferably at least one of the anti-oxidant additives may be selected from the group consisting of aromatic amines or phenolic compounds, such as hindered phenols.

The lubricating oil may comprise one or more corrosion inhibitor additives which might be used, at least in part, to replace ZDDP. Preferably, the corrosion inhibitor additives may be selected from conventional non-sulphur detergent additives.

The lubricating oil may comprise one or more other additives which may be known to one skilled in the art as lubricating oil additives. Such additives may include one or more of anti-foam additives, Viscosity Index improvers and dispersants.

The invention will now be illustrated with respect to the following Examples, and the figures, in which:

FIG. 1 shows the particulate emissions by mass (in g/k Wh) according to the standard ECE Reg. 49 test, for combinations of low and high sulphur fuels (LSF and HSF), and low and high sulphur lube oils (LSL and HSL), in the presence and absence of the CRT.

FIG. 2 shows the data for total particulate emissions (number/k Wh) for combinations of low and high sulphur fuels, and low and high sulphur lube oils measured using both a Scanning Mobility Particle Sizer (SMPS) and an Ultrafine Particulate Monitor (UPM).

EXAMPLES

Tests were performed on a Heavy Duty (HD) diesel engine (11 litre (21/cyl), turbo-charged/intercooled diesel engine fitted with electronic fuel injection equipment)

Two different fuels were tested. Fuel 1 was a low sulphur fuel comprising 10 ppm sulphur and corresponding to EN-590 specification. Fuel 2 was a high sulphur fuel and was produced by doping a sample of fuel 1 to 50 ppm sulphur.

Two lubricants were tested. The first was a conventional lube oil comprising 0.75 wt % sulphur, supplied by Castrol, herein designated as “high sulphur”. The second was a low sulphur synthetic based SAE 5W-30 lube oil comprising 0.14 wt % sulphur, in which the ZDDP level was reduced compared to the conventional lube oil, to give a ZDDP level of 0.38 wt %, and oleamide was added as an additional antiwear additive.

Tests were performed both with and without a Continuously Regenerating Trap (CRT), supplied by Johnson Matthey.

Particle size measurement was made with both a TSI 3071 Scanning Mobility Particle Sizer (SMPS) (scanning between 7-320 nm), and a Booker Systems Ultrafine Particulate Monitor (UPM) (giving total particle count>3 nm)

Tests were performed under the ECE Reg. 49 testing conditions. For engines built prior to 2000 this is the standard homologation test for heavy duty diesel engines in Europe.

The R49 test cycle requires the engine to be tested over 13 steady-state modes at based at different speed/load operating conditions. The emissions in each mode are measured and aggregated according to a regulated procedure to give a single result for the cycle. For particle emissions the standard test method measures the mass of particles produced in each mode. The result therefore gives an aggregated total mass of particles produced per k Wh of power.

In the examples given, the total number of particulate emissions was measured using both a standard Scanning Mobility Particle Sizer (SMPS) (scanning between 7-320 nm), and an Ultrafine Particulate Monitor (UPM) (giving total particle count >3 nm). These results were then aggregated to give a combined mode particle emission value for the R49 cycle in number of particles per k Wh. The aggregation was performed in the same manner as for the regulated procedure for mass of particulate emissions the R49 test.

For comparison, FIG. 1 gives the particle emissions measured as particle mass (in g/k Wh) according to the standard ECE Reg. 49 test, for combinations of the low and high sulphur fuels (LSF and BSF), and the low and high sulphur lube oils (LSL and HSL), in the presence and absence of the CRT.

It can be seen that in the absence of a CRT the emissions, in terms of particle mass, are approximately similar. Significant changes in mass emission in the absence of the trap would not be expected as only a small proportion of the sulphur in the fuel is emitted as particulates, and the changes in sulphur level will have only a small impact on regulated emissions. However in the presence of the CRT, due to the presence of the oxidation catalyst, the total mass of particles produced is more dependent on the sulphur levels in the lube oil and fuel and reduces as the sulphur levels in the lube oil and fuel are decreased.

FIG. 2 shows the data for total particle emission rates (number/k Wh) for the 10 ppm and 50 ppm sulphur fuels with the two lubricants measured using both SPMS and UPM. The two bars for each set represent repeat experiments showing high reproducibility.

The shaded bars represent the SMPS measurement and the clear bars represent the UPM measurement, the difference between the shaded bars and the open bars being the small particles detected by the UPM (but not the SMPS) i.e. nucleation mode particles of between about 3 and 7 nm diameter.

It can be seen that with the 50 ppm sulphur fuel and high sulphur lube oil then essentially all the accumulation mode particles are removed from the emissions by the presence of the trap (CRT), but a larger number of nucleation mode particles are emitted compared to the test in the absence of the CRT. This increase is at least in part, dud to reaction of sulphur dioxide on the oxidation catalyst in the CRT to produce sulphates, which are emitted from the CRT under the conditions in certain modes of the R49 test.

For a low sulphur fuel with the high sulphur lube oil it can be seen that in the absence of a trap the total particle emissions are very similar to those for the high sulphur fuel, as may be expected by comparison with FIG. 1. Again this, is due to the fact that in the absence of the trap only a small proportion of the sulphur in the fuel is emitted as particulates. In the presence of a trap, essentially all of the accumulation mode particles are removed from the emissions, as seen for the high sulphur fuel. In this case the total number of nucleation mode particles produced decreases compared to the high sulphur fuel.

For the low sulphur lube oil with a low sulphur fuel the emissions in the absence of the CRT are again similar to those seen for the experiments with the high sulphur lube oil and the low sulphur and high sulphur fuels respectively, as expected. However the use of a low sulphur lube oil with a low sulphur fuel in the presence of the CRT gives total particulate emissions that are very significantly lower than expected based on the reduction in the sulphur level.

In particular the use of a low sulphur lube oil in combination with a low sulphur diesel fuel leads to a reduction in the emissions of nucleation mode particles from a diesel engine fitted with a particulate trap. 

1. A method of reducing the number of nucleation mode particles in the emissions from a heavy duty diesel engine fitted with a catalyzed particulate trap, which is a continuously regenerating trap (CRT™) comprising both an oxidation catalyst and a particulate trap, which method comprises lubricating a heavy duty diesel engine with a lubricating oil consisting essentially of an anti-wear, anti-oxidant and corrosion-inhibiting lubricating oil having a low sulphur content of less than 0.4% by weight and comprising ZDDP and optionally at least one additional additive selected from the group consisting of an anti-wear additive, an anti-oxidant additive, a corrosion inhibitor, an anti-foam additive, a Viscosity Index improver and a dispersant, wherein ZDDP is present at a concentration of up to 0.4 percent by weight, and employing a fuel having a low sulphur content of below 50 ppm by weight, to thereby reduce the emissions of nucleation mode particles from the heavy duty diesel engine fitted with a catalyzed particulate trap, wherein the nucleation mode particles have a diameter in the range of between about 3 nm and 7nm.
 2. A method according to claim 1, wherein the sulphur content (by weight) of the fuel is below 20 ppm.
 3. A method according to claim 2, wherein the sulphur content (by weight) of the fuel is 10 ppm or lower.
 4. A method according to claim 1, wherein the low sulphur lubricating oil has a sulphur content (by weight) of less than 0.3%.
 5. A method according to claim 4, wherein the low sulphur lubricating oil has a sulphur content (by weight) of less than 0.2%.
 6. A method according to claim 5, wherein the low sulphur lubricating oil has a sulphur content (by weight) of less than 0.15%.
 7. A method according to claim 1, wherein the low sulphur lubricating oil comprises one or more anti-wear additives selected from the group consisting of (a) molybdenum containing compounds, (b) organic based friction modifiers, and (c) salicylate-type detergents.
 8. A method according to claim 1, wherein the low sulphur lubricating oil comprises one or more anti-oxidant additives selected from the group consisting of aromatic amines and phenolic compounds.
 9. A method according to claim 1, wherein the low sulphur lubricating oil comprises one or more corrosion inhibitor additives selected from the non-sulphur detergent additives.
 10. A method according to claim 1, wherein the low sulphur lubricating oil comprises one or more other additives selected from one or more of anti-foam additives, Viscosity Index improvers and dispersants.
 11. A method according to claim 2 wherein the low sulphur lubricating oil has a sulphur content (by weight) of less than 0.3%.
 12. A method according to claim 3 wherein the low sulphur lubricating oil has a sulphur content (by weight) of less than 0.3%.
 13. A method according to claim 2 wherein the low sulphur lubricating oil has a sulphur content (by weight) of less than 0.2%.
 14. A method according to claim 3 wherein the low sulphur lubricating oil has a sulphur content (by weight) of less than 0.2%.
 15. A method according to claim 2 wherein the low sulphur lubricating oil has a sulphur content (by weight) of less than 0.15%.
 16. A method according to claim 3 wherein the low sulphur lubricating oil has a sulphur content (by weight) of less than 0.15%.
 17. A method according to claim 2, wherein the low sulphur lubricating oil comprises one or more anti-wear additives selected from the group consisting of (a) molybdenum containing compounds, (b) organic based friction modifiers, and (c) salicylate-type detergents.
 18. A method according to claim 2, wherein the low sulphur lubricating oil comprises one or more anti-oxidant additives selected from the group consisting of aromatic amines and phenolic compounds.
 19. A method according to claim 2, wherein the low sulphur lubricating oil comprises one or more corrosion inhibitor additives selected from the non-sulphur detergent additives.
 20. A method according to claim 2, wherein the low sulphur lubricating oil comprises one or more other additives selected from one or more of anti-foam additives, Viscosity Index improvers and dispersants.
 21. A method according to claim 3, wherein the low sulphur lubricating oil comprises one or more anti-wear additives selected from the group consisting of (a) molybdenum containing compounds, (b) organic based friction modifiers, and (c) salicylate-type detergents.
 22. A method according to claim 3, wherein the low sulphur lubricating oil comprises one or more anti-oxidant additives selected from the group consisting of aromatic amines and phenolic compounds.
 23. A method according to claim 3, wherein the low sulphur lubricating oil comprises one or more corrosion inhibitor additives selected from the non-sulphur detergent additives.
 24. A method according to claim 3, wherein the low sulphur lubricating oil comprises one or more other additives selected from one or more of anti-foam additives, Viscosity Index improvers and dispersants.
 25. A method according to claim 14, wherein the low sulphur lubricating oil comprises one or more anti-wear additives selected from the group consisting of (a) molybdenum containing compounds, (b) organic based friction modifiers, and (c) salicylate-type detergents.
 26. A method according to claim 14, wherein the low sulphur lubricating oil comprises one or more anti-oxidant additives selected from the group consisting of aromatic amines and phenolic compounds.
 27. A method according to claim 14, wherein the low sulphur lubricating oil comprises one or more corrosion inhibitor additives selected from the non-sulphur detergent additives.
 28. A method according to claim 14, wherein the low sulphur lubricating oil comprises one or more other additives selected from one or more of anti-foam additives, Viscosity Index improvers and dispersants.
 29. A method according to claim 16, wherein the low sulphur lubricating oil comprises one or more anti-wear additives selected from the group consisting of (a) molybdenum containing compounds, (b) organic based friction modifiers, and (c) salicylate-type detergents.
 30. A method according to claim 16, wherein the low sulphur lubricating oil comprises one or more anti-oxidant additives selected from the group consisting of aromatic amines and phenolic compounds.
 31. A method according to claim 16, wherein the low sulphur lubricating oil comprises one or more corrosion inhibitor additives selected from the non-sulphur detergent additives.
 32. A method according to claim 16, wherein the low sulphur lubricating oil comprises one or more other additives selected from one or more of anti-foam additives, Viscosity Index improvers and dispersants. 